What is it?

In experimental science the basic answer always has to be "It's a thing that looks like this". At least The Cat will be happy

Yes, but what is it? The Cat in Red Dwarf. Photograph: Ronald Grant Archive

Generational divides in particle physics can to some extent be traced by whether you tend to quote Monty Python, The Hitchhiker's Guide, or Red Dwarf.

Pretty often in Red Dwarf the crew try to explain to The Cat what some exotic transdimensional plot device is; his response is usually to keep asking "What is it?" until the explanation is completely phenomenological. Usually a multicoloured swirly thing in space*.

In the end proving something exactly will be impossible. "Exactly" is a statement which requires infinite precision. You can never know exactly how long a piece of string is. The question is really "Are its properties consistent with it being a Higgs boson or not?" and one has to decide how many different things have to be consistent, to what precision, before one starts calling it a Higgs.

The Standard Model is remarkably predictive when it comes to the Higgs boson. It predicts that it exists, for a start. It does not predict its mass very well, but once the mass is fixed, everything else follows. For example, a Standard Model Higgs must have zero electrical charge, and zero "spin" (that is, it carries no intrinsic angular momentum). There also is a precise prediction of what particles it will produce when it decays. The whole point of the Higgs mechanism is to give particles mass, and this means the heavier a particle is, the more likely it is to be produced in a Higgs decay**.

At present of course we have no significant signal. However we do have sensitivity, such that the data could have been inconsistent with the existence of a Higgs boson, down to about 130 GeV in Higgs mass. But they remain consistent. This statement (even if statistically stronger) relies mainly on comparisons to predicted event rates. We also know something about the distribution of the events, but there is no tell-tale mass peak since we have two neutrinos produced, which we cannot measure. We also have no measurement of the other properties expected of the Higgs boson. So it will be quite a weak statement even if the statistical uncertainty shrinks as we get more collisions.

To start calling this, or some future effect if this one goes away, a "Higgs boson" rather than an "excess" or a "candidate", we would need to measure more of its properties. I think the main thing here is to see the whatever-it-is (assuming it is something) decay in at least two, and preferably more, different ways. If the relative decay rates look as expected for a Higgs boson, it becomes pretty compelling, progressively more so as more of these decay modes are accumulated. The more we measure, the more the discussion will move from "excess" to "Higgs boson candidate" to "Higgs boson" to "Standard Model Higgs boson". Or not, of course, if the data show something else.

But in the end, the answer to "What is it?" in physics is always "It's a thing that looks like this." Go, Cat.

For the record, I belong to the Hitchhiker's Guide generation really, despite being a Red Dwarf fan. But I can't resist adding this link from the prehistory of CERN communications, which is squarely from the Monty Python era:

I keep expecting to hear the new generation start quoting the Big Bang Theory, but either that's just too self referential (after all, it quotes us) or I just don't go to enough parties anymore.

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